1. Field of the Invention
The present invention relates to a charging member and a charging device of contact charging type suitably used with an electrophotographic image forming apparatus.
2. Related Background Art
Conventionally, for example, in image forming apparatuses of electrophotographic or electrostatic type, as a charging device for uniformly charging (including removal of electricity) an image bearing body such as an electrophotographic photosensitive body or an electrostatic recording dielectric body, a corona charger (corona discharger) has been used.
The corona charger is a charging device of non-contact type and has a discharging electrode such as a wire electrode and a shield electrode surrounding the discharging electrode. The corona charger is disposed so that a discharging opening portion is opposed an image bearing body (to be charged) in a non-contact manner, so that a surface of the image bearing body is charged with predetermined potential by exposing the surface of the image bearing member to discharge current (corona shower) generated by applying high voltage to the discharging electrode and the shield electrode.
Recently, as a charging device for charging a body to be charged (such as an image bearing body), many charging devices of contact type have been proposed and put to practical use, since they have advantages of less ozone and low power consumption in comparison with the corona chargers.
In the charging device of contact type, an electrically conductive charging member of roller type (charging roller), fur-brush type, magnet brush type or blade type is contacted with a body to be charged such as an image bearing body, and, by applying predetermined charging bias to the charging member (charging member of contact type, charger of contact type; referred to as "contact type charging member" hereinafter), the surface of the body to be charged is charged with predetermined polarity and potential.
A charging mechanism of contact charging (mechanism of charging, charging principle) including two kinds of charging mechanisms, i.e., (1) discharge charging mechanism and (2) injection charging mechanism, and, independence upon preferential mechanism, various properties are realized.
(1) Discharge Charging Mechanism
In this system, the surface of the body to be charged is charged by a discharging phenomenon caused in a small gap between the contact type charging member and the body to be charged.
Since the discharge charging mechanism has predetermined threshold values for the contact type charging member and the body to be charged, voltage greater than charging potential must be applied to the contact type charging member. Further, although a creating amount of discharge product is considerably small in comparison with the corona charger, since creation of the discharge product cannot be avoided in principle, a bad influence of active ions such as ozone cannot be avoided.
(2) Injection Charging Mechanism
In this system, the surface of the body to be charged is charged by directly injecting electrical charges from the contact type charging member to the body to be charged. This is also referred to as "direct charging" or "injection charging" or "electrical charge injecting charging".
More specifically, a contact type charging member having middle resistance is contacted with the surface of the body to be charged, and the electrical charges are directly injected on the surface of the body to be charged, without a discharging phenomenon, i.e., without using the discharging fundamentally. Thus, even if the voltage applied to the contact type charging member is smaller than the discharging threshold value, the body to be charged can be charged to potential corresponding to the applied voltage. Since the injection charging mechanism does not generate ozone, there is no bad influence of discharge product.
However, due to injection charging, contacting ability of the contact type charging member against the body to be charged greatly influences upon the charging ability. Therefore, the contact type charging member must be made more compact, a difference in speed between the contact type charging member and the body to be charged must be increased, and the contact type charging member must be contacted with the body to be charged more frequently.
The Inventors have proposed a new charging system for effecting the injection charging via electrically conductive particles, as described in U.S. patent application Ser. Nos. 09/035,109, 09/035,108 and 09/035,022, all filed Mar. 5, 1998.
In this charging system, a holding amount of electrically conductive particles can be increased by using a member having a foam body (foam material) layer as a charging member.
However, in the foam material, since a resistance value is varied with the number of cells in the surface of the foam, unevenness of charging potential apt to occur in accordance with the number of cells.